Cavity Wall Window Opening

[Willmott85]

Evening all,

I am in the process of renovating a bathroom in a 1930's property and would like to improve the insulation around the window.

I have found a number of cavity closers online (Kingspan Kooltherm, Manthorpe, Therma-Pro etc) but they are all fixed widths.  

Given my cavity varies (not hugely) but is mainly 50mm, I was wondering if I could use some left over Celotex PIR board to form the cavity closer?  I would be able to cut the PIR to the desired width to fit tight in the cavity.  

The only issue I see is moisture bridging, as there will not be a 'built in'DPC as seen in the specialist cavity closers mentioned above.  

Would foil facing all edges of the PIR be good enough to ensure no moisture/damp travels from the outer to inner leaf of the wall?

Once the cavity closer is fitted (whatever it may be) the window opening will be faced with 6mm Hardiebacker board, before the wooden window surround is fitted.

Any advice greatly recieved.

 

[s60tsy]

use some dpc membrane between the (kingspan) board and the wall if youre worried

 

[Willmott85]

Hi s60tsy,

How far past the PIR board should the DPC membrane go?

Also is it required to be installed round the corner of outer leaf brick at the opening?

I have read that you need to use different DPC membrane for vertical jams and sill. Is this really the case?